Many different types of electrical devices are powered by low voltage systems. Such devices include, but are not limited to, household and kitchen products, power tools, outdoor lawn and garden equipment, lighting systems, camping accessories, automotive products and a variety of battery chargers. Many of these devices are operated in areas where conventional A.C. sources, or high level D.C. sources, are available. In such instances, it would be advantageous to use the available power sources to operate the devices. Consequently, facility must be provided for converting the available power source to a source and level compatible with the power operating level of the device to be used.
In the past, many systems and techniques have been employed to accomplish the necessary conversion. A basic technique used in the past to convert conventional high A.C. voltage to low D.C. voltage included the use of a step-down transformer and a rectifier. However, such conventional A.C. sources typically operate at low frequencies which necessitates the use of a bulky transformer of substantial weight and size. In this instance, the physical parameters of the transformer prohibit feasible maneuverability of any system which includes the bulky transformer and the device being powered. Therefore, the versatility of any system utilizing the bulky transformer is extremely limited.
Another converter system has employed a technique whereby the initial power source, either A.C. or D.C., is converted to a high frequency signal by use of a pair of alternately switched transistors. Voltage reduction is then accomplished by a high frequency transformer. The transformer output is then rectified to obtain the low level D.C. voltage for operation of a low voltage device.
One example of such a circuit has a series leg which includes an inductor, a series capacitor and the primary of a high frequency transformer. One end of the series leg is connected between a pair of alternating switches such as transistors with parallel-connected diodes. The other end of the series leg is connected between a pair of split capacitors. The other ends of the split capacitors and the other ends of the switches are connected across a full wave rectifier.
This type of circuit requires the split capacitors which function as supply sources on alternate halves of the circuit operation and may require a filter capacitor. With this type of circuit, it is difficult to directly sense information which could be determined by the voltage parameter of the series capacitor. Further, it is not possible to sense directly the transformer current. Therefore, current sensing would have to be accomplished by some other indirect means such as, for example, a current transformer.
Examples of circuits using systems similar to the split capacitor system are illustrated in U.S. Pat. Nos. 4,196,469; 4,227,243 and 4,333,134.
In another type of system, such as that disclosed as prior art (FIG. 1) in U.S. Pat. No. 4,353,112, the high frequency principle is employed and includes an oscillatory circuit with an inductor and a capacitor. The inductor is connected to the primary of a high frequency transformer or it could be a leakage inductance of the high frequency transformer. Oscillations developed by the oscillatory circuit control a switching means to connect a D.C. supply to the inductor which receives and stores energy from the supply. During the oscillatory operation of such systems, power may be coupled to a load circuit only during a portion of the total cycle time which results in using either a separate inductor or a transformer with substantial leakage inductance and a complex control circuit such as described in U.S. Pat. No. 4,353,112. In this system, the topology can't be used on a transformer with a very low leakage inductance because transient imbalance in transformer volt-seconds can permanently saturate the transformer and has to be recovered by complex control means. When the transformer goes into permanent saturation without recovery, the switching transistors see excessive currents which could cause a catastrophic failure unless complex control means are used.
Also, there are many other systems which involve the complex principle of operation as noted above. Such systems include an oscillatory circuit which depends on a complex circuit design utilizing many reactive components, transistors, integrated circuit chips, diodes and resistive components. While the basis for high frequency operation permits the use of a lightweight transformer, the number of discrete components and chips employed in the complex circuit design lessen significantly any space advantage gained by use of the lightweight transformer.
Consequently, the space requirement for the many components, chips and transformer limits the adaptability of these prior systems for use as a power source in conjunction with devices where space and power requirements are critical factors. Therefore, there remains a need for a voltage converter of simple design which requires relatively little space but provides the necessary and efficient power during total cycle time of converter operation to operate powered devices.
Such a converter would enhance the versatility and portability of the powered devices by occupying a very limited space while not adding significantly to the bulk and weight of the combination of the powered device and converter.